Coming into contact with an atmosphere containing moisture, copper generally changes in color over the years from its original reddish orange to brown, dark brown, blackish brown, and patina, until a natural patina is formed on the surface of copper. The natural patina produced on copper plate in that manner is formed of such compounds as basic copper sulfate, basic copper carbonate, and basic copper chloride, individually or in combination. This patina constitutes a stable and tightly bonded protective film, hard to dissolve in water. Patinized copper is suitable for roofing and other uses where a high degree of weather resistance is required. Moreover, the patina film produces excellent design effects on the appearance of copper roofing and is very important in improving the aesthetic appearance of copper articles.
However, it takes a long time to produce a natural patina. Often it will be 20 to 30 years before a natural patinated film has sufficiently developed.
Meanwhile, artificially patinated copper plates such as one prepared by subjecting the surface of the copper plate to a chemical coloring treatment with chemical solutions and then forming a resin layer on the copper plate surface for coating the same have been proposed and put to practical use.
The problem with such chemically-colored copper plates is that because of a large difference in extensibility between the copper base and the resin layer, the chemically-colored copper material is very poor in formability and is not suitable for use as roofing, where such vigorous bending as folding is required. In other words, when subjected to bending, for example, the resin layer will peel off because of the difference in the amounts of extension and compression between the two layers.
There is another problem with the chemically-colored copper material. Because of poor adhesion between the copper base and the artificial patinated layer, there is a possibility that part of the artificial patina will peel off or fall off in a relatively short time, namely before the natural patina has grown enough. At the places where the artificially patinated layer peels off or falls off, the surface of the base copper where no natural patina has grown is exposed. The exposed copper surface takes on the brown color peculiar to copper oxide, greatly spoiling the outward appearance of the whole copper material.